Experts say this tiny field is emblematic of a fundamental change in the way U.S. companies deal with risky work and in OSHA's ability to hold companies accountable when workers are injured or die.

Over the past decade, considerable attention has focused on the out-sourcing of U.S. jobs to foreign countries with lower safety standards and wages. Much less noticed has been the trend of companies out-sourcing their dirtiest, most perilous work within the U.S.

Since 2003, tower climbing has ranked among the most dangerous jobs in America, compiling an average annual death rate more than 10 times that of construction work. Almost 100 climbers have been killed on the job, 50 of them on cell sites.

From their perch atop the contracting chain, carriers typically set many of the crucial parameters for work on cell sites, including deadlines, pay rates and even technical specifications, down to the exact degree an antenna should be angled. An analysis of cell tower deaths by ProPublica and PBS "Frontline" showed that tight timetables and financial pressure often led workers to take fatal shortcuts or to work under unsafe conditions.

"We've had a number of situations where we think that accidents were caused by companies trying to meet deadlines and cutting corners on safety in order to meet those deadlines," said Jordan Barab, OSHA's deputy administrator.

But Barab said it's difficult for the agency to hold cell companies responsible for safety violations involving subcontractors. In most cases, federal officials have interpreted OSHA regulations to mean that carriers can be held accountable only if they exercised direct control over subcontractors' work or were aware of specific unsafe conditions.

OSHA has not sanctioned cell carriers for safety violations implicated in any subcontractor deaths on cell sites since 2003, a review of agency records by ProPublica and PBS "Frontline" found.

Since 2003, tower climbing has ranked among the most dangerous jobs in America, compiling an average annual death rate more than 10 times that of construction.

The agency attempted to fine a carrier just once and failed, losing a nearly three-year legal battle with a regional cell company in Kentucky. The agency has never taken on the four major carriers Verizon, T-Mobile, AT&T and Sprint even though there have been almost two dozen fatalities on jobs done for their networks.

Most of OSHA's enforcement efforts have focused on a transient cast of small subcontractors, though they, too, typically have eluded significant penalties. Over the last nine years, the median fine levied for safety violations linked to a fatal tower accident was $3,750, an analysis by ProPublica and PBS "Frontline" showed.

OSHA has made little effort to connect the deaths of tower workers to specific carriers. Barab acknowledged that OSHA officials had not known until ProPublica and PBS "Frontline" told them that there have been 15 fatalities on AT&T jobs since 2003 more than at the other three major carriers combined over the same period.

If that many of a company's own employees suffered workplace fatalities, Barab said, it would be "exactly the kind of case" the agency would consider for its Severe Violator Enforcement Program, which OSHA reserves for employers who show an indifference to safety. But because tower climbers are subcontractors, the agency examines each death separately, and does not connect them to the carriers whose cell networks they build.

"Generally, we can only cite employers when their employees are at the work site," Barab said. "As you go up the line, it becomes much more difficult to actually hold the companies at the top responsible."

Officials at the four major cell carriers declined requests to be interviewed for this story. In a written statement, AT&T said its contracts require the companies it hires for tower work to adhere to OSHA safety regulations. Statements from Verizon, T-Mobile and Sprint called subcontractors' safety a priority.

"We have always maintained a strong safety policy and record for our employees, which we also expect from our contractors," the T-Mobile statement said.

Some labor experts counter that OSHA's limited enforcement authority gives companies like cell carriers a powerful incentive to avoid any role in worker safety. If those at the top bear no consequences, they said, unsafe practices by subcontractors will continue.

"Until there's an incentive for the carriers to actually improve this they're probably not going to," said Catherine Ruckelshaus, legal co-director at the National Employment Law Project, a liberal group that advocates for worker rights. If regulators held carriers accountable for accidents involving contract workers, "they're going to try to insure that the subcontractors that they engage are legitimate, above-board subcontractors with sophistication and good health and safety training."

But Ed Reynolds, an industry consultant who was AT&T's president of network services until 2007, said punishing carriers for tower-climbing accidents would not reduce fatalities.

"You can take the captain of the ship approach," he said. "You can say that Randall Stephenson is responsible because Randall Stephenson's the CEO of AT&T. But what impact would [that] have on the eventual safety of future crews? I think that's too far to connect."

The use of subcontractors and temporary employees has been on the rise for several decades as businesses have sought flexibility, relief from union requirements, and savings.

An analysis of labor statistics by the Government Accountability Office found that the number of workers employed by contractors jumped nearly 25 percent from 1995 to 2005 and the total number of "contingent" workers including contractors, temporary employees, self-employed workers, part-time workers and day laborers totaled more than 40 million.

The trend toward subcontracting has changed the American workplace in ways unanticipated by the authors of the Occupational Safety and Health Act, said Judson MacLaury, who served as the Department of Labor's historian from 1972 to 2006.

"Subcontracting was not a factor in the development of the Act," he said. "It only became an issue for OSHA itself when it became an enforcement problem."

The shift toward subcontracting has been most noticeable in high-risk industries such as oil and gas, trucking, nuclear waste removal and home-building. Though workplace injuries and deaths have decreased over OSHA's 40-year lifespan, subcontract workers face greater risks than traditional employees, studies have shown. Contractors were injured or killed in eight of the National Council for Occupational Safety and Health's 10 worst workplace accidents of 2010. In six of the accidents, no full-time employees died, only contractors.

To sanction companies for safety violations involving subcontractors, OSHA must meet the requirements of what it calls the multi-employer citation policy. It's a directive spelled out in agency rules, rather than the law.

The policy, which dates to OSHA's early days, was created with construction and manufacturing in mind, industries with centralized work sites at which the supervisors for the companies that own and operate the sites are more likely to be present.

The tower industry doesn't function this way. Work is done on thousands of cell sites in remote locations all over the country for short periods of time. Carriers sometimes don't own towers, leasing space for their antennas instead, and typically don't have employees on site.

On paper, the multi-employer policy allows OSHA to cite companies that contract out work if they supervise a work site and can correct safety violations or require others to do so.

In some instances, courts and OSHA's appeals commission have interpreted the policy more narrowly, ruling that if companies didn't expose their own employees to danger or didn't have specific knowledge of the conditions that caused harm, OSHA could not impose sanctions.

Business organizations, such as the National Association of Home Builders, have argued against penalties based on the multi-employer policy, contending that employers should be responsible only for their own employees, not those hired by subcontractors.

OSHA sometimes wins such battles. In 2009, for example, the 8th U.S. Circuit Court of Appeals in Missouri upheld the agency's citation against a general contractor for failing to ensure that a subcontractor used proper safety gear on a scaffold. But these victories often have been costly and time-consuming, playing out over years.

Some of the agency's leaders have been skeptical about pursuing cases under the multi-employer policy. Charles Jeffress, OSHA's top administrator from 1997 to 2001, said companies whose employees are not on site should not be held accountable for hazards created by subcontractors.

"You can't sit in an office building thousands of miles away and direct the work," he said.

Philip Colleran, who worked as a senior compliance officer at OSHA in Illinois for 17 years before leaving to start a private consulting firm in 1990, said the agency has become reluctant to initiate multi-employer cases without what it considers overwhelming evidence.

"The sad truth is the agency's lawyers are still reluctant to prosecute even clear-cut controlling entities, such as general contractors," he wrote in an email, "let alone nebulous relationships such as the type of subcontractors you're addressing."

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The one instance in which OSHA tried to sanction a carrier after one of its subcontractors died on a cell tower illustrates the legal hurdles the agency must clear.

"The employees all confirmed the fact that that was just normal practice. That's just the way they normally did things."

The case began in May 2006, at a cell tower in rural Kentucky.

When Randy Gray, an investigator with Kentucky OSHA, arrived on the site, 22-year-old Michael Sulfridge was lying face up, eyes open, 380 feet below where he had been working. Sulfridge looked curiously unaffected by the fall aside from the blood streaked under his nose. His scuffed gray sneaker and work bag, holding only a pack of cigarettes, dangled from a barbed wire fence next to his body.

Sulfridge's co-workers readily acknowledged that he had been free climbing working without fall protection gear and that this was pervasive on the job. They told Gray they routinely didn't wear straps, known as lanyards, to clip themselves onto the tower.

"The lanyards were all in the back of the supervisor's truck," Gray said. "Some of them were even in new packaging, never opened up. The employees all confirmed the fact that that was just normal practice. That's just the way they normally did things."

Free climbing is strictly forbidden under OSHA regulations, but tower climbers sometimes do it to work more quickly. Gray cited Tower Services Inc., the subcontractor that employed Sulfridge, for several safety violations and proposed $143,000 in fines.

Then he went a step further, taking aim at Bluegrass Cellular, the cell company that had hired Tower Services.

In his report, Gray said Bluegrass' field operations manager, Daniel Combs, told him he regularly visited about half of the carrier's towers when work was going on. Tower Services had worked on Bluegrass sites for at least two years, Gray learned. He concluded that Combs "could have detected" that Tower Services climbers weren't taking proper safety precautions.

"It just seems logical that whenever the carrier was here, that they had the possibility to know that these people were not tying off with personal protective equipment," Gray said.

Gray issued citations against the cell carrier for failing to conduct safety inspections and ensure the use of fall protection gear, proposing $7,000 in fines.

Bluegrass pushed back. In letters from its lawyers and affidavits from employees, the company said it had no responsibility for the safety of subcontractors.

"Bluegrass is convinced that only by exerting actual control over the work of an independent contractor could an owner ever have any duty to oversee the safety of a contractor's employees," the carrier's attorney wrote in a 2007 letter to the assistant general counsel for the Kentucky Labor Cabinet. "Bluegrass has never exerted actual control over the work of an independent contractor doing construction work at any of its towers."

Combs submitted an affidavit saying that he hadn't visited half of Bluegrass' cell sites, as Gray's report had said, and only "sometimes" checked on their progress.

Ultimately, Gray couldn't prove that anyone from Bluegrass had been on the site when Sulfridge fell or had witnessed the unsafe practices that led to his death. In 2009, OSHA dismissed the citations against the carrier.

Bluegrass and Tower Services declined to comment for this story. OSHA reduced Tower Services' fine to $24,000 after the company promised to provide more training, require employees to use fall protection and facilitate random inspections of its sites.

Gray, who retired from OSHA in 2008 and runs a safety consulting company, came away frustrated with the outcome.

"The easiest and simplest way to avoid getting a citation is to do what Bluegrass did, and don't go to the job site," he said.

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With the exception of the Sulfridge case, OSHA has mostly targeted small subcontractors for discipline when tower climbers were killed in accidents.

But even companies hit with multiple citations often received deep discounts on fines based on their size or lack of previous safety violations, OSHA records show. Some obtained additional reductions by appealing or found ways to sidestep penalties altogether. OSHA waived a $6,300 fine issued after a tower climber sustained massive injuries in a 240-foot fall when it could not find the company owner, Ryan Chapman, who had shut down his business and started a new one. Contacted by ProPublica and PBS "Frontline," Chapman said he was unaware of OSHA's efforts to reach him.

"I figure if they want to talk to me, they'd find me," he told a reporter. "I mean, you got my number." (We tracked down Chapman's phone number on an online industry message board.)

With about 800 tower-climbing subcontractors operating nationwide many of them small and short-lived it's unlikely that pressing these companies will bring systemic safety improvements, said David Weil, a Boston University economics professor who studies subcontracting.

"It's like the old game of Whac-a-Mole," he said. "You can enforce your OSHA standards on that individual contractor and hit the mole. But there are a lot of other contractors that are going to pop up."

In 2006 when 19 tower climbers died—10 of them working on cell sites—OSHA tried a new approach to improve safety in the industry. It launched a partnership with the National Association of Tower Erectors, the trade group for tower companies, aimed at reducing injury and fatality rates and raising awareness of key worksite hazards. As part of the initiative, the agency offered discounts on regulatory fines and NATE members agreed to audit their own safety practices.

The early results looked promising: 85 NATE members joined in the first year, performing almost 600 self-audits and sending more than 1,000 workers for additional training.

"It was great," said Gordon Lyman, a safety expert at WesTower, a large tower company, and a member of NATE's OSHA relations committee. Jim Coleman, NATE's chairman, said in a written response to questions from ProPublica and PBS "Frontline" that the partnership helped change the industry's relationship with OSHA from adversarial to collaborative.

But there was a sticking point between the agency and the group, correspondence and email obtained under the Freedom of Information Act shows.

NATE leaders wanted cell carriers, tower owners and general contractors to participate in the initiative. In a Dec. 20, 2006 e-mail to OSHA, Patrick Howey, then the executive director of NATE, said companies that hired subcontractors with poor safety records were "a major source of accidents."

"It's like the old game of Whac-a-Mole. You can enforce your OSHA standards on that individual contractor and hit the mole. But there are a lot of other contractors that are going to pop up."

It's unclear if OSHA or NATE formally invited the major cell companies to participate in the safety partnership, but they were welcome to join. None did. In its statement, AT&T said it strongly supported the partnership, which the carrier credited with "a dramatic improvement in worker safety," but declined to say why it did not participate. T-Mobile did not respond to questions about the partnership. In an email, a Verizon spokesman said the company had no record of being invited to join. A statement from Sprint said the company did not participate because "we are not in the business of erecting towers."

Rob Medlock, OSHA's Cleveland area office director and an agency point man for the NATE partnership, said carriers had little incentive to get involved as they were already outside of regulators' reach.

"It puts you in the ballgame, where right now you're outside the park," said Medlock, who left OSHA in 2010.

As cell companies raced to expand their networks in 2008, 12 climbers died, eight of them on cell towers. Three worked for companies enrolled in the OSHA-NATE safety initiative. The relationship between OSHA and NATE began to sour.

In a September 2008 conference call, OSHA officials told NATE leaders that the group needed to "step up" by hiring a safety specialist, conducting its own accident investigations and creating an accident database, agency meeting minutes show.

Howey fired back in a Dec. 4, 2008 memo, arguing that it was OSHA and the companies on top of the contracting chain who needed to step up.

Carriers and others who demand "unrealistic scheduling" or hire unsafe contractors are "one of the biggest factors in tower site fatalities and must be addressed," Howey wrote. "OSHA needs to find a way to deal with these companies or accept that fatalities are going to continue."

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There's much debate among federal regulators and industry experts on how OSHA should enforce safety standards more effectively in industries like tower climbing, which rely heavily on subcontracting.

Some, including several former OSHA officials, say Congress needs to change the law that created the agency more than 40 years ago to expand the definition of "employer" to include companies that contract out work.

"We have laws that are structured in a way that no longer look like the workplaces that they're trying to regulate," said Weil, the economics professor.

Medlock and others said OSHA could create rules to achieve the same goal administratively.

John Henshaw, OSHA's administrator from 2001 to 2004, said any attempt to broaden the agency's authority would face stiff opposition from businesses, whose challenges could clog up the court system. Instead, he suggested that simply making it known which accidents are connected to which cell carrier would encourage companies to ensure that subcontractors address safety problems.

"The American public can put pressure on those cell carriers, and ultimately only do business with the ones who are doing it the right way," he said.

For that to happen, OSHA would have to start systematically collecting information on contracting relationships when tower workers die. Barab said regulators faced obstacles in trying to do this.

"It's a lot of work to try to trace things up to the ultimate owner," he said. "We're talking about sometimes multiple levels here."

Still, ProPublica and PBS "Frontline" was able to obtain this data for fatalities since 2003, sometimes just by making a few phone calls, and Barab conceded that OSHA could do the same.